In the field of RF signal transmission, it is known that the output impedance of an RF signal source should match the input impedance of its load as closely as possible. The better the match, the greater the efficiency of the power transfer is between the source and the load. Theoretically, if the impedances are exactly matched, the load absorbs 100% of the signal transmitted from the source. On the other hand, when there is a difference between the load impedance and the impedance of the source, the load does not fully absorb the entire signal from the source and “reflects” a portion of the signal back toward the source.
In a conventional RF transmission system, a transmitter is connected to an antenna through a device normally referred to as a matching network, or antenna coupler. The function of the matching network is to transform the impedance of the antenna to the desired characteristic impedance of the transmitter.
If the elements of such a transmission system remained constant, the coupler could be fixed and would not need to be changed. However, in practice, there are many factors which vary the impedance of the various elements in the transmission system, and thus, the impedance of the antenna coupler may also be changed. For example, the characteristics of antennas may change over time as the antennas endure wear and tear.
In addition, in mobile RF transmission systems, the characteristic impedance of an antenna may vary considerably with changes in the natural surroundings of the antenna. Further, the impedance of the antenna and other elements in a transmission system may be dependent upon the frequency of the signal being transmitted. Thus, mobile frequency hopping transmission systems encounter many changes in the characteristic impedance of the elements of the system for which it may be helpful to change the operation of the matching network between the transmitter and its associated antenna.
Conventional systems use differing techniques to identify when the matching network should be changed, and to identify what changes needed to be made. For example, it is known to measure the Voltage Standing Wave Ratio (“VSWR”) of a transmitting system. If the VSWR exceeded a predetermined threshold, such as 2.5, the elements of the coupler could be altered to reduce the VSWR. Because the VSWR measures the ratio between the forward and reflected signals, it is considered a measure of the relative “goodness” of the impedance matching, a high VSWR indicating a relatively poor match and, accordingly, a relatively high reflected signal. A high VSWR therefore wastes power in a portable device and reduces the radiated signal magnitude.
Exemplary advances in devices to measure the impedance of a load, which are helpful in communications devices including matching networks, have been made. For example, U.S. Pat. Nos. 5,386,194 and 5,206,600 to Moehlmann, assigned to the same assignee as the present invention, disclose an impedance detector that samples the voltage and the current of a signal passing through a transmission line, and derives the impedance parameters independently of the amplitude or phase of the signal.
U.S. Pat. Pub. 2008/0266021 to Van Bezooijen et al. discloses a communications device including a power amplifier coupled to an antenna via a matching network. An impedance detector is coupled to the input of the matching network. The impedance detector measures the impedance at the input of the matching network. A control unit is coupled to the impedance detector and compares the impedance at the input of the matching network with a reference value representing the nominal load line impedance, and adjusts the matching network accordingly.
The control unit of this system, however, may require many iterations in changes until the impedance is properly matched. Consequently, new mobile wireless communications devices employing new methods of matching antenna load impedances to amplifier impedances may be needed.